Reserved Sectors on a Physical Volume

A physical volume reserves the first 128 sectors to store various types of DASD configuration and operation information. The /usr/include/sys/hd_psn.h file describes the information stored on the reserved sectors. The locations of the items in the reserved area are expressed as physical sector numbers in this file, and the lengths of those items are in number of sectors.

The 128-sector reserved area of a physical volume includes a boot record, the bad-block directory, the LVM record, and the mirror write consistency (MWC) record. The boot record consists of one sector containing information that allows the read-only system (ROS) to boot the system. A description of the boot record can be found in the /usr/include/sys/bootrecord.h file.

The boot record also contains the pv_id field. This field is a 64-bit number uniquely identifying a physical volume. This identifier might be assigned by the manufacturer of the physical volume. However, if a physical volume is part of a volume group, the pv_id field will be assigned by the LVM.

The bad-block directory records the blocks on the physical volume that have been diagnosed as unusable. The structure of the bad-block directory and its entries can be found in the /usr/include/sys/bbdir.h file.

The LVM record consists of one sector and contains information used by the LVM when the physical volume is a member of the volume group. The LVM record is described in the /usr/include/lvmrec.h file.

The MWC record consists of one sector. It identifies which logical partitions might be inconsistent if the system is not shut down properly. When the volume group is varied back online for use, this information is used to make logical partitions consistent again.